I'll give it a shot:
1: The Return - Build Me a Reason: This is their debut album as far as I'm concerned since it was actually on a label and actually... exists. I can't get enough of this band, I've been nuts over them ever since the summer of 2003. So obsessed that countless hours have gone into searching for their older work (listed on their site). It's hard to find a bad song (though, it does seem to take people a few listens to really feel attatched) on the album. Will it be there defining work? I don't know. They have a new album coming out soon (i.e. whenever) but the songs I've heard off of it have been spectacular. Best part about relentlessly promoting this band is that I can link a free sample of the album off. =[
2: Frou Frou - Details: Gorgeous voice + great lyrics = great sound. I'm not usualy one for electronica (yet it's made it onto this list twice =[) but Frou Frou's talent and voice is just amazing. I found this album by extreme chance and I have never been disapointed with it once. She's gotten some popularity lately, being on two major movie soundtracks (Shrek 2 and Garden State) which hopefully means she'll release a followup album.
3: Anberlin - Blue Prints for the Blackmarket: As far as I know, this was their debut and man, what a powerful debut. So many great songs, I don't know if this will become their defining album but I think they have alot of potential to do better things. Though, I think I've listened to this album so much over a long period of time that no matter how good followup is it may just never touch it.
4: The Postal Service - Give Up: This has sort of become the token album in almost every list of albums I've made in the past 4 months, which is funny because I really didn't want to fall in love with it. More electronica, showing that I'm a massive hypocrite even more. Like Frou Frou's Details, Ben Gibbard's side project exhibits great (some of the best) lyrics and a great voice. Some bad songs, but the great ones really soar.
5: The Shins - Oh, Inverted World: I'm not sure when they stopped being Flake Music or what the whole situation with that was. But the Shins' debut album is undeniably great but not so great that it becomes their defining work. No matter how great their debut was, Chutes Too Narrow shattered it.